The electron microscope (EM) has been a mainstay for study of fastidious gastroenteritis viruses. Although 2nd and 3rd generation tests have been developed for the detection of "Norwalk" viruses and rotaviruses. EM remains an essential tool: (1) as the "supreme court" when newer tests yield variable results, (2) in the quest for new agents of viral gastroenteritis, (3) for visualizing the site of attachment of monoclonal antibodies, (4) for serologic studies and direct visualization of particles. A fastidious 27 mm virus of epidemic gastroenteritis--the Marin County agent--was administered to volunteers in an attempt to generate particle positive stools. Particles were detected by EM in stools of one ill volunteer. The identification of the particle as the Marin County virus was established by IEM; the particle was reacted with paired sera from the original outbreak and a significant antibody rise was detected. In other studies, the site of attachment on the rotavirus particle of monoclonal antibody with subgroup specificity was studied by direct and indirect IEM. Finally, about 50 percent of the episodes of pediatric diarrhea are still without any known etiology. A major effort will be made to examine by IEM or indirect IEM such "negative" specimens in an attempt to detect new etiologic agents.